Viking 1 was the first of two spacecraft (along with Viking 2) sent to Mars as part of NASA's Viking program.[2] It was the first spacecraft to successfully land on Mars and perform its mission,[3] and held the record for the longest Mars surface mission of 2307 days[2] or 2245 sols[2] (from landing until surface mission termination, Earth time) until that record was broken by the Opportunity Rover on May 19, 2010.[4]

Following launch using a Titan/Centaur launch vehicle on August 20, 1975 and a 10-month cruise to Mars, the orbiter began returning global images of Mars about 5 days before orbit insertion. The Viking 1 Orbiter was inserted into Mars orbit on June 19, 1976 and trimmed to a 1513 x 33,000 km, 24.66 h site certification orbit on June 21. Landing on Mars was planned for July 4, 1976, the United States Bicentennial, but imaging of the primary landing site showed it was too rough for a safe landing. The landing was delayed until a safer site was found. The lander separated from the orbiter on July 20 08:51 UTC and landed at 11:53:06 UTC. It was the first attempt by the United States at landing on Mars.

The instruments of the orbiter consisted of two vidicon cameras for imaging (VIS), an infrared spectrometer for water vapor mapping (MAWD) and infrared radiometers for thermal mapping (IRTM).[5] The orbiter primary mission ended at the beginning of solar conjunction on November 5, 1976. The extended mission commenced on December 14, 1976 after solar conjunction. Operations included close approaches to Phobos in February 1977. The periapsis was reduced to 300 km on March 11, 1977. Minor orbit adjustments were done occasionally over the course of the mission, primarily to change the walk rate — the rate at which the areocentric longitude changed with each orbit, and the periapsis was raised to 357 km on July 20, 1979. On August 7, 1980 Viking 1 Orbiter was running low on attitude control gas and its orbit was raised from 357 × 33943 km to 320 × 56000 km to prevent impact with Mars and possible contamination until the year 2019. Operations were terminated on August 17, 1980 after 1485 orbits.

The lander and its aeroshell separated from the orbiter on July 20 08:51 UTC. At the time of separation, the lander was orbiting at about 5 km/s. The aeroshell's retrorockets fired to begin the lander deorbit maneuver. After a few hours at about 300 km altitude, the lander was reoriented for atmospheric entry. The aeroshell with its ablative heat shield slowed the craft as it plunged through the atmosphere. During this time, entry science experiments were performed by using a retarding potential analyzer, a mass spectrometer, and pressure, temperature and density sensors.[5] At 6 km altitude, traveling at about 250 m/s, the 16 m diameter lander parachutes deployed. Seven seconds later the aeroshell was jettisoned, and 8 seconds after that the three lander legs were extended. In 45 seconds the parachute had slowed the lander to 60 m/s. At 1.5 km altitude, retrorockets on the lander itself were ignited and, 40 seconds later at about 2.4 m/s, the lander arrived on Mars with a relatively light jolt. The legs had honeycomb aluminum shock absorbers to soften the landing.[5]

The landing rockets used an 18-nozzle design to spread the hydrogen and nitrogen exhaust over a large area. NASA calculated that this approach would mean that the surface would not be heated by more than one degree Celsius, and that it would move no more than 1mm of surface material. Since most of Viking's experiments focused on the surface material a more straightforward design would not have served.

The Viking 1 Lander touched down in western Chryse Planitia ("Golden Plain") at 22°41′49″N48°13′19″W﻿ / ﻿22.697°N 48.222°W﻿ / 22.697; -48.222 at a reference altitude of −2.69 km relative to a reference ellipsoid with an equatorial radius of 3397.2 km and a flatness of 0.0105 (22.480° N, 47.967° W planetographic) at 11:53:06 UT (16:13 local Mars time). Approximately 22 kg of propellants were left at landing.

Transmission of the first surface image began 25 seconds after landing and took about 4 minutes. During these minutes the lander activated itself. It erected a high-gain antenna pointed toward Earth for direct communication and deployed a meteorology boom mounted with sensors. In the next 7 minutes the second picture of the 300° panoramic scene (displayed below) was taken.[6] On the day of the landing, the Lander sent back the first picture taken from the surface of Mars. On the day after the landing the first color picture of the surface of Mars (displayed below) was taken. The seismometer failed to uncage, and a sampler arm locking pin was stuck and took 5 days to shake out. Otherwise, all experiments functioned normally. The lander had two means of returning data to Earth: a relay link up to the orbiter and back, and by using a direct link to Earth. The data capacity of the relay link was about 10 times higher than the direct link.[5]

The lander had two facsimile cameras, three analyses for metabolism, growth or photosyntheses, a gas chromatograph-mass spectrometer (GCMS), an x-ray fluorescence spectrometer, pressure, temperature and wind velocity sensors, a three-axis seismometer, a magnet on a sampler observed by the cameras, and various engineering sensors.[5]

The Viking 1 Lander was named the Thomas Mutch Memorial Station in January 1982 in honor of the leader of the Viking imaging team. The lander operated for 2245 sols (about 2306 Earth days or 6 years) until November 11, 1982 (sol 2600), when a faulty command sent by ground control resulted in loss of contact. The command was intended to uplink new battery charging software to improve the lander's deteriorating battery capacity, but it inadvertently overwrote data used by the antenna pointing software. Attempts to contact the lander during the next four months, based on the presumed antenna position, were unsuccessful.[7] In 2006 the Viking 1 lander was imaged on the Martian surface by the Mars Reconnaissance Orbiter.[8]

Samples resembled soil produced from weathering of basaltic lavas.[citation needed] The tested soil contained abundant silicon and iron, along with significant amounts of magnesium, aluminum, sulfur, calcium, and titanium. Trace elements, strontium and yttrium, were detected. The amount of potassium was one fifth the average for the Earth's crust. Some chemicals in the soil contained sulfur and chlorine that were like those remaining after the evaporation of sea water. Sulfur was more concentrated in the crust on top of the soil than in the bulk soil beneath. The sulfur may be present as sulfates of sodium, magnesium, calcium, or iron. A sulfide of iron is also possible.[9] The rovers Spirit and the Opportunity both found sulfates on Mars.[10]Opportunity (landed in 2004 with advanced instruments) found magnesium sulfate and calcium sulfate at Meridiani Planum.[11] Using results from the chemical measurements, mineral models suggest that the soil could be a mixture of about 90% iron-rich clay, about 10% magnesium sulfate (kieserite?), about 5% carbonate (calcite), and about 5% iron oxides (hematite, magnetite, goethite?). These minerals are typical weathering products of mafic igneous rocks.[12] All samples heated in the gas chromatograph-mass spectrometer (GSMS0] gave off water. However, the way the samples were handled precluded an exact measurement of the amount of water. But, it was around 1%.[13] Studies with magnets aboard the landers indicated that the soil is between 3 and 7 percent magnetic materials by weight. The magnetic chemicals could be magnetite and maghemite. These could come from the weathering of basalt rock.[14][15] Experiments carried out by the Mars Spirit Rover (landed in 2004) indicated magnetite could explain the magnetic nature of the dust and soil on Mars. Magnetite (which is very dark) was found in the soil and the most magnetic part of the soil was dark.[16]

Viking carried a biology experiment whose purpose was to look for evidence of life. The Viking spacecraft biological experiments weighed 15.5 kg (34 lbs) and consisted of three subsystems: the Pyrolytic Release experiment (PR), the Labeled Release experiment (LR), and the Gas Exchange experiment (GEX). In addition, independent of the biology experiments, Viking carried a Gas Chromatograph/Mass Spectrometer (GCMS) that could measure the composition and abundance of organic compounds in the martian soil.[17] The results were surprising and interesting: the GCMS gave a negative result; the PR gave a negative result, the GEX gave a negative result, and the LR gave a positive result.[18] Viking scientist Patricia Straat recently stated, "Our (LR) experiment was a definite positive response for life, but a lot of people have claimed that it was a false positive for a variety of reasons."[19] Most scientists now believe that the data were due to inorganic chemical reactions of the soil; however, this view may be changing after the recent discovery of near-surface ice near the Viking landing zone. Some scientists still believe the results were due to living reactions. No organic chemicals were found in the soil. However, dry areas of Antarctica do not have detectable organic compounds either, but they have organisms living in the rocks.[20] Mars has almost no ozone layer, unlike the Earth, so UV light sterilizes the surface and produces highly reactive chemicals such as peroxides that would oxidize any organic chemicals.[21] The Phoenix Lander discovered the chemical perchlorate in the Martian Soil. Perchlorate is a strong oxidant so it may have destroyed any organic matter on the surface.[22] If it is widespread on Mars, carbon-based life would be difficult at the soil surface.

High-precision test of general relativity by the Cassini space probe (artist's impression)

Gravitational time dilation is a phenomenon predicted by the theory of General Relativity whereby time passes differently in regions of different gravitational potential. Scientists used the lander to test this hypothesis, by sending radio signals to the lander on Mars, and instructing the lander to send back signals. Scientists then found that the observed signals matched the predictions of the theory of General Relativity;[23] see Shapiro delay.